The leading cause of death for people aged 18-45, civilian and military, is hemorrhagic shock following traumatic blood loss. Early detection of hemorrhage is essential to give medical staff an opportunity to intervene. When there are no external signs of trauma, however, autonomic compensatory changes cause internal bleeding to be very difficult to detect. Undetected internal blood loss can be manifest by compartment scale fluid shifts, surface fluid redistribution and a change in blood composition as the body compensates for net blood volume loss. The absolute blood hematocrit changes but other physiological measures also change and, depending on the measurement site and primary hemorrhage site, some change earlier. The earliest indicators of hemorrhage are well known but there are no means to obtain a continuous real-time record in order to log temporal changes and reactions to external probing/examination. There remains an urgent need for a process to allow determination of hematocrit noninvasively and in vivo. This invention teaches a noninvasive method and apparatus for obtaining a continuous time record of relative blood hematocrit, particularly in humans.